Cancer cells undergo dramatic changes in carbohydrate expression during the onset and progression of disease. Detection of these changes is essential for understanding their basic biology as well as for monitoring changes for diagnostic purposes. Carbohydrate expression is monitored almost exclusively with anti-carbohydrate antibodies and lectins. In addition, anti-carbohydrate antibodies and lectins can be used to target tumors. For these proteins to provide accurate information on the expression of carbohydrate tumor antigens or to reliably target tumors, they must bind specifically to a particular carbohydrate of interest. Unfortunately, analysis of specificity has been difficult with conventional tools and most antibodies and lectins have not been fully characterized. In spite of the lack of information, many of the antibodies and lectins are thought to be specific and conclusions regarding carbohydrate expression are made based on this perception. We are using a carbohydrate microarray to evaluate the specificity of many commonly used anti-carbohydrate antibodies and lectins. To date, we have analyzed 26 antibodies and found 40% have specificity problems. The lack of specificity can lead to inaccurate basic science information, inaccurate diagnostic information, and inefficient tumor targeting agents.